1. Field of the Invention
This invention relates in general to processes and apparatus for biologically treating a liquid pollution stream, and more particularly to processes and apparatus for increasing the bioremediation rate of a liquid pollution stream in a bioreactor.
2. Prior Art
It is common in various manufacturing processes to form as a by-product liquid streams which must then be disposed. These liquid streams will in many cases contain environmentally undesirable compounds, or pollutants. For example most organic compounds adversely effects the chemical oxygen demand (COD) levels of rivers, streams, lakes, ponds, harbors, oceans and/or underground water reservoirs. These organic compound levels are generally determined by measuring the biological oxygen demand (BOD) level which is a subset or component of the COD level. Such streams can not be discharged into rivers or other water reservoirs without first reducing or eliminating these pollutants in order to achieve environmentally acceptable COD or BOD levels.
Traditionally, processes used to treat various solid, liquid or gas pollution streams have been divided into four general categories: physical, chemical, biological, and thermal. Of these, biological treatment processes have been considered to potentially be the most attractive alternative to the other categories of treatment processes or at least as an attractive adjunct to the other categories of treatment processes. The primary reason being they detoxify and/or chemically degrade many pollutant streams to environmentally "safe" compounds; namely, carbon dioxide and water or methane, without the introduction or production of other pollutants into the environment. However, use has been limited in large part by their inefficiency, their high operating costs, and the limited known pollution streams which they can effectively treat.
These biological treatment processes are commonly referred to as bioremediation processes. Bioremediation encompasses all detoxifying or chemical degradation treatments of pollutants with aerobic and/or anaerobic microorganisms. It is common for a pollutant stream to exist which naturally contains many different microorganisms, either through adaptation, random mutation or the induction of degradative pathways, all of which may consume the pollutants as an energy or food source. This consumption by the microorganisms generally results in a natural detoxification and/or chemical degradation of the pollutants. Usually the number of such microorganisms increases in relation to the amount of pollutants present. Even when these microorganisms do not derive energy from a pollutant, they often will co-metabolize and degrade it along with another food source. This has been found to be the basis of some important degradative pathways for recalcitrant materials.
Bioremediation processes have generally been grouped into two broad areas: land treatment and liquid treatment. The liquid is commonly water. In land treatment, solids, sludge or liquid pollution are mixed into surface soils or composted. Although land treatment is one of the least expensive of the treatments it does require adequate amounts of land which many not be available. Land treatment processes are also now facing more stringent restrictions imposed by the Resource Conservation & Recovery Act (RCRA) and the Comprehensive Environmental Response, Compensation & Liability Act (known as "Superfund").
In water treatment processes, bioreactors are employed to increase the rate of biodegradation of pollution in liquid streams and can be a more rapid and efficient means of degrading pollution than any of the other treatment processes.
The term "bioreactor" as used herein includes any structure having a cavity that could hold a liquid pollution stream. This would include natural structures such as ponds, lakes, swamps, rivers, streams, harbors, and oceans, as well as man-made structures such as tanks, pipes and other storage vessels. It would also include onetime flow through digesters, such as onetime flow through activated sludge digesters, aeration basins, and ponds.
The simplest bioremediation processes entail pumping the liquid pollution stream into a bioreactor, generally a pond or holding tank, and letting the stream remain in the bioreactor until the desired bioremediation has naturally occurred. In most instances this process requires either a commercially unacceptable period of processing time or reactor size, or the desired level of bioremediation can not be obtained. Efforts to improve upon such natural bioreactors have been made. The most common of these is the injection of large amounts of air or oxygen into the bioreactor.
In bioremediation processes microorganisms are categorized into two major groups (aerobes and anaerobes) based upon their ability to use oxygen in energy generation. Aerobes have the ability to use oxygen in energy generation. Because of this ability the addition of oxygen or air to a bioreactor can rapidly increase the aerobic populations in the bioreactor. This increase in population will result in an increased rate of remediation of pollutants consumed by aerobes in the bioreactor. Unfortunately, these greatly increased populations of aerobes become sludge. The greater the population the more sludge that will be formed which because of its composition contributes to COD and BOD levels in the bioreactor mostly transforming soluble COD and BOD to solid COD and BOD. For this reason they must be removed prior to discharge of the treated stream into the environment. This removed sludge then has to be dried and stored in landfills or recycled into fertilizer, compost, etc. Another problem is that aerobic remediation of the stream is limited by the amounts of oxygen that can be added to the bioreactor. In conventional aeration systems the aerobes will consume 25 to 30 pounds of BOD related compounds per day per horsepower used in aerating the bioreactor. In more effective, but more expensive, diffused injection systems, this consumption rate can be increased to 75 to 150 pounds of BOD related compounds per day per horsepower. The significant capital cost associated with the equipment necessary to provide the aeration, as well as the continuous energy consumption and equipment maintenance cost associated with operating the aeration equipment are highly undesirable features of such systems. The magnitude of these costs can be seen when one considers that for a typical pulp and paper mill the resulting processing water stream requires the daily treatment of 16.6 million gallons to eliminate 100,000 pounds of BOD related compounds. To achieve this amount of consumption by aerobes, 3,000 to 6,000 horse power is utilized in conventional aeration systems. This will require annually 19-39 million kilowatts of electricity. At present day energy costs this would be approximately $75,000-$200,000 per month. This does not include the maintenance and chemical costs associated with the operation of this system. Such cost may run an additional $70,000 per month.
Anaerobes, unlike aerobes, are generally unable to use oxygen in energy generation and in fact are harmed by oxygen rich environments. Holding ponds, lagoons, and any other bodies of water with reduced dissolved oxygen levels are examples of bioreactors which utilize anaerobic bioremediation processes. However, it has been difficult to achieve significant increases in anaerobic populations. Therefore, even though it is known that anaerobic bioremediation processes produce considerably less sludge than aerobic processes, anaerobic bioremediation processes generally have been considered to be less efficient than aerobic bioremediation processes. In some bioremediation processes nutrients, trace elements and sometimes ex-situ grown microorganisms have been added to the biodigester. It is possible to isolate and purify pollutant specific microbes and reproduce these in relatively small numbers in fermentation units (ex-situ grown organisms). After careful analysis of the pollution and many treatability studies, mixes of the fermented microbes are blended and added to bioreactors to increase the rate and effectiveness of the bioremediation process. Often this treatment process requires the repeated introduction of the microorganisms in relative large quantities on a daily basis for extended periods of time. Both the isolation, purification, fermentation and introductions are expensive and many times are less than effective. Improvements are needed in finding, isolating and determining the mix of microbes best suited for the then ambient bioreactor environment and in recognizing when the mix needs to change to respond to bioreactor environmental changes.
Other pollution treatment processes have been designed to make use of both aerobes and anaerobes for treatment of pollution streams. In such dual treatment processes it is common, because of efficiency and economic reasons (sludge reduction and less BOD load on the aerobic system), to first anaerobically treat the pollution stream under anaerobic remediation process conditions before aerobically treating it under aerobic remediation process conditions. An anaerobic holding area is arranged upstream of the aerobic holding area. In those cases where the bioreactor is a deep pond the upper section may be capable of acting as the aerobic holding area and the lower section because of substantially reduced oxygen content may be capable of acting as the anaerobic holding area. Even though the anaerobic treatment is usually less rapid it does not produce the quantities of sludge aerobic systems produce and the desired amount of BOD removal can be accomplished by simply keeping the pollution stream in the bioreactor long enough for the desired reduction to occur. Such anaerobic treatment generally removes a significant quantity of the undesirable material from the pollution stream, thus resulting in less volume of pollution to be aerobically treated and thus reducing the overall cost of treatment. This is a desirable treatment strategy whenever enough storage area is available to achieve the necessary retention times.
Other recent effects to improve the rate of anaerobic remediation processes have been developed which utilize secondary treatment bioreactors. These are commonly referred to as anaerobic digesters or activated sludge anaerobic digesters. These digesters generally consist of containers filled with rocks, activated sludge, plastic beads or plant matter such as bagasse or reeds. The purpose of this fill material is to provide sites at which the anaerobic microorganisms may become attached or immobilized, but which still allows the pollutant stream passing through the digester to contact the attached or immobilized anaerobic microorganisms.
In these anaerobic bioreactor/digester processes the pollution stream is retained in the digester for the time needed to achieve a desired BOD reduction in the stream and then discharged either directly into the environment, or more commonly to an aerobic treating area where the anaerobically treated pollution stream is contacted with selected aerobic microorganisms under aerobic remediation process conditions until the desired additional BOD reduction in the stream is achieved. In this latter embodiment the anaerobic bioreactor/digester process acts as a modified form of the dual treatment processes described above.
Another problem associated with current bioreactor designs is that microorganisms tend to wash out of the bioreactors. As the microorganisms leave the bioreactor the rate of bioremediation, and therefore its effectiveness, is generally reduced.
Still another problem associated with remediation processes employing bioreactors is their sensitivity to environmental conditions within the pollution stream being introduced to the bioreactor or to environmental changes within the bioreactor. Rapid changes in the environment or the pollution stream are likely to destroy or greatly reduce (upset) the degradation efficiency of most, if not all, of the microorganisms in a bioreactor. It is known that microorganisms hibernate or slow down when the usually narrow environment for which they are acclimated is not present. However, microorganisms have proven extremely adaptable over time and many consortiums have evolved for most every biodegradable compound and environment. For example, temperature decreases cause microorganisms that were active at the higher temperatures to decrease or stop their activity and microorganisms better adapted to the new temperature to become active. Unfortunately, this deactivating and activating of microorganisms can be very slow, sometimes taking upwards of six months.
In addition to temperature changes other examples of environmental changes that can cause these upset conditions include pH and oxygen level of the stream, or introduction of poisons or biocides to the stream, and/or concentration or mix changes of the pollutants. Present known efforts have not been able to satisfactorily overcome these problems.